We have conducted radiocarbon (14C) dating of Japanese tree rings from 1053 to 921 BCE and 41 BCE to 130 CE. Dating was also performed using oxygen isotope dendrochronology to investigate subtle structures of the calibration curve corresponding to the beginning and the end of the Yayoi period in Japan. These two results followed IntCal20, which included the 14C ages of two Japan-sourced trees. The findings suggest that dating of specimens obtained from areas around the Japanese archipelago may be affected by periodic monsoons from the ocean, an effect that needs further examination.

We present two new millennium-long tree-ring oxygen isotope chronologies for central and northern Japan, based on 9693 annually resolved measurements of tree-ring oxygen isotopes from 39 unearthed samples consisting mainly of Japanese cedar (Cryptomeria japonica). These chronologies were developed through cross-dating of tree-ring widths and δ18O data from multiple samples covering the periods 2349–1009 BCE (1341 yr) and 1412–466 BCE (947 yr) for central and northern Japan, respectively. In combination with our published chronology for central Japan, the tree-ring δ18O dataset currently available covers the past 4354 yr (2349 BCE to 2005 CE), which represents the longest annually resolved tree-ring δ18O dataset for Asia. Furthermore, the high-resolution temporal record of 14C contents independently developed by Sakurai et al. (2020) was reproduced by our 14C measurements of earlywood and latewood in annual rings for the period 667–660 BCE.

Regional offsets from Northern Hemisphere radiocarbon (14C) calibration curves are widely recognized for monsoon Asia and often hinder accurate 14C dating. In this paper, we explore the possible linkage between summer monsoon intensity and 14C offsets using tree-ring δ18O and 14C data from Thailand. We developed a 297-yr floating tree-ring δ18O chronology comprising seven teak log-coffin samples from the Ban Rai rock shelter site, northwestern Thailand. The outermost ring of our chronology was estimated to date from 358–383 CE, within a 95.4% (2σ) probability range, based on a total of 10 14C measurements that were wiggle-matched against a mixed calibration curve evenly weighted from the IntCal20 and SHCal20 curves. Backward trajectory analysis showed that an intensified (weakened) summer monsoon detected in a modern tree-ring δ18O chronology was most likely to be induced by increased (decreased) air mass transport from the tropical Indian Ocean, which is an area of intense upwelling where the 14C concentration is lower than the atmospheric 14C level. However, partly because of the limited sample size and dating uncertainty, the direct linkage between the tree-ring δ18O series and 14C records obtained from our teak log-coffin samples could not be statistically verified.

We present a long-term seasonal tree ring cellulose oxygen isotope (δ18Oc) time series created by analyzing four segments (S1, S2, S3, and S4) per year during the period of 1951–2009 from southeastern Tibetan Plateau. This intraseasonal δ18Oc reveals the onset and mature phase of the summer monsoon precipitation in this region. Analysis indicates that the δ18Oc of S1 has the strongest correlation with precipitation during the regional monsoon onset (29–33 pentads, May 21–June 10, r = −0.69), and the δ18Oc values for S2, S3, and S4 correlate strongly with June, July, and August precipitation, respectively. Combined δ18Oc of S2, S3, and S4 shows the most robust correlation (r = −0.82) with the mature-phase monsoon precipitation (June-July-August, JJA), passing rigorous statistical tests for calibration and verification in dendroclimatology. These results demonstrate the feasibility in using long-term intraseasonal δ18Oc to reconstruct the Asian summer monsoon's intraseasonal variations.

This study carried out accelerator mass spectrometry radiocarbon (AMS 14C) measurement of Japanese tree rings dating from the middle to early modern eras to investigate calibration curve fine structure. Tree-ring ages were determined by dendrochronology or δ18O chronology for Japanese trees. 14C ages from the 15th century to the middle of the 17th century followed the IntCal13 calibration curve within measurement error. Different patterns of fluctuations during the latter half of the 17th century to the early the 18th century were observed in different tree samples. In the 19th century, patterns of 14C ages of different samples appeared similar but did not exactly match each other.

We collected pore waters using an in situ pore water-squeezer for a submersible Shinkai 2000 at six depths beneath the sediment surface within a deep-sea “cold seep” giant clam community off Hatsushima Island, Sagami Bay, Japan. A box core sample was also collected ca. 4.5 km east of the community and pore waters were separated. Dissolved inorganic carbon (DIC) was extracted and purified in a vacuum line and 14C concentration was determined with a Tandetron accelerator mass spectrometer at Nagoya University after conversion to graphite targets using a batch Fe-catalytic hydrogen reduction method. ∆14C values decreased with increasing depth to −938‰ at the sulfate concentration minimum. This indicates that methane used for the active reduction of sulfate and formation of hydrogen sulfide, which is used by symbiotic chemoautotrophic bacteria in gills of the giant clams, is almost dead and is likely supplied from the deep. ∆14C values of DIC vary linearly with δ13C values along a mixing line between that in the bottom water and that produced by the oxidation of dead methane. The δ13C value of DIC oxidized from dead methane is estimated to be ca. −45‰.